This paper aims to elucidate retention characteristics of some pharmaceuticals and personal care products (PPCPs), and endocrine disrupting chemicals (EDCs), by two polyamide low pressure reverse osmosis (LPRO) membranes. Feed solution pH did not have an influence on rejections of undissociated solutes, which was most likely governed by adsorption, size exclusion and diffusion simultaneously. Size exclusion was presumably dominant, especially with tight membranes (UTC-70U). Rejections of the solutes with low dipole moment (<1.0 debye) decreased with increasing octanol-water partition coefficient (K(ow)). The solutes with large K(ow) values were most likely adsorbed on membrane and subsequently passed through it resulting in larger diffusion coefficient (D(p)). The rejections decreased with increasing D(p) values irrespective of their dipole moments. Rejections of solutes with comparatively larger dipole moments might be dominated by diffusion and/or convection rather than their hydrophobicity. However, rejections of solutes with hydroxyl and carboxyl functional groups by UTC-60 increased with solution pH. More than 80% rejections were obtained for degree of dissociation (alpha)>0.5. Electrostatic repulsion played a key role for rejection of dissociated solutes, especially by loose LPRO membranes. Therefore, assessing the dissociation degree at desired pH values can be a key step to obtain an insight of rejection mechanisms by polyamide membranes.

A new generation of reverse osmosis membranes, low-pressure reverse osmosis (LPRO) membranes, have been developed for operation under very low pressure (below 0.5 MPa). LPRO membranes have received attention especially for their application in the field of water and wastewater treatment, to provide a high water flux at low operating pressure while maintaining very good rejection levels of salts and organics. Our previous work on LPRO has shown that the rejection of some inorganic salts and organic compounds depends appreciably on the pH of the bulk solution, probably because LPRO membranes can have an electric charge. In this study we investigated experimentally the effectiveness of different LPRO membranes in separating inorganic salts and organic compounds from a bulk solution with different pH conditions. We also tried to measure membrane ξ-potential by using a streaming potential method. The results indicated that the membrane ξ-potential as well as the ion size or molecular weight of organics can be considered key factors in the rejection of ionic salts and dissociated organic compounds including pesticides and endocrine disruptors.

The coexisting natural organic matters (NOM) in the water environment are expected to influence rejections of endocrine disrupting chemicals (EDCs) in membrane filtration. However, such influences and rejection mechanisms are not well understood. This paper aims to elucidate on the rejection of three representative EDCs: Bisphenol A (BPA), 17β–estradiol (E2) and 4–Nonylphenol (NP) by a LPRO membrane (UTC–60) with and without three representative NOM: humic acid (HA), fulvic acid (FA) and effluent organic matters (EfOM) in solutions by laboratory experiments. The EDCs rejections were enhanced with increasing solution pH and negative membrane zeta potential as a result of decreased membrane pore size. Adsorption of the EDCs on membrane surface and size exclusion were the most likely rejection mechanisms in absence of the NOM. Coexistence of the NOM in solutions, in general, enhanced the EDCs rejections. However, no specific trends in rejection were observed possibly due to diffusion of the adsorbed NOM and EDCs molecules through the membrane pores, physicochemical properties of the EDCs and several types of NOM molecules present in EfOM. The EDCs adsorption on NOM molecules and their adsorption on the membrane surface and/or retention by size exclusion were the most likely rejection mechanisms.